The description of prior art will be primarily based on the list of references presented at the end of this section.
Concerns over the safety of earlier lithium secondary batteries led to the development of lithium ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials as the anode. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1. In order to minimize the loss in energy density due to this replacement, x in LixC6 must be maximized and the irreversible capacity loss Qir in the first charge of the battery must be minimized. The maximum amount of lithium that can be reversibly intercalated into the interstices between graphene planes of a perfect graphite crystal is generally believed to occur in a graphite intercalation compound represented by LixC6 (x=1), corresponding to a theoretical specific capacity of 372 mAh/g [Ref. 1].
In addition to carbon- or graphite-based anode materials, other inorganic materials that have been evaluated for potential anode applications include metal oxides, metal nitrides, metal sulfides, and a range of metals, metal alloys, and intermetallic compounds that can accommodate lithium atoms/ions. In particular, lithium alloys having a composition formula of LiaA (A is a metal such as Al, and “a” satisfies 0<a<5) has been investigated as potential anode materials. This class of anode material has a higher theoretical capacity, e.g., Li4Si (3,829 mAh/g), Li4.4Si (4,200 mAh/g), Li4.4Ge (1,623 mAh/g), Li4.4Sn (993 mAh/g), Li3Cd (715 mAh/g), Li3Sb (660 mAh/g), Li4.4Pb (569 mAh/g), LiZn (410 mAh/g), and Li3Bi (385 mAh/g). However, for the anodes composed of these materials, pulverization (fragmentation of the alloy particles or thin films) proceeds with the progress of the charging and discharging cycles due to expansion and contraction of the anode during the absorption and desorption of the lithium ions. The expansion and contraction also tend to result in reduction in or loss of particle-to-particle contacts or contacts between the anode material and its current collector. These adverse effects result in a significantly shortened charge-discharge cycle life.
To overcome the problems associated with such mechanical degradation, several approaches have been proposed, including (a) using nano-scaled particles of an anode active material, (b) composites composed of small electrochemically active particles supported by less active or non-active matrices or coatings, and (c) metal alloying [e.g., Refs. 2-13]. Examples of active particles are Si, Sn, and SnO2. However, most of prior art composite electrodes have deficiencies in some ways, e.g., in most cases, less than satisfactory reversible capacity, poor cycling stability, high irreversible capacity, ineffectiveness in reducing the internal stress or strain during the lithium ion insertion and extraction cycles, and some undesirable side effects.
For instance, as disclosed in U.S. Pat. No. 6,007,945 [Ref. 3], a solid solution of titanium dioxide and tin dioxide was utilized as the anode active substance in the negative electrode of a rechargeable lithium battery. The density of the negative electrode made was 3.65 g/cm3, and the reversible capacity of the negative electrode containing TiO2—SnO2 in a ratio of 39:61 by weight, was found to be 1130 mAh/cm3. This was equivalent to 309.6 mAh/g, although the obtained rechargeable lithium battery was calculated to have energy density of 207 watt-hour per liter. Furthermore, the nano particles of the anode material react with the electrolyte during the charge-discharge cycles, resulting in reduced long-term utility.
As described in U.S. Pat. No. 6,143,448 [Ref. 4], a composite was formed by mixing carbon with a metal salt in water, followed by evaporation, heating, and further treatment. The process produces a composite with many pores, which are not always preferred. The best achievable capacity was reported to be in the range of 750-2,000 mAh/cm3. With a density of 4 g/cm3, this implies a maximum capacity of 500 mAh/g
In U.S. Pat. No. 7,094,499 [Ref. 5], Hung disclosed a method of forming a composite anode material. The steps include selecting a carbon material as a constituent part of the composite, chemically treating the selected carbon material to receive nano particles, incorporating nano particles into the chemically treated carbon material, and removing surface nano particles from an outside surface of the carbon material with incorporated nano particles. A material making up the nano particles forms an alloy with lithium. The resulting carbon/nano particle composite anodes did not exhibit any significant increase in capacity, mostly lower than 400 mAh/g.
Clerc, et al., in U.S. Pat. No. 6,524,744 [Ref. 6], proposed a multiphase material that comprises a ceramic matrix material having one or more of Sn, Sb, Bi, Pb, Ag, In, Si and Ge nano-dispersed in the matrix. The ceramic matrix is based upon carbides, nitrides and oxides of group IV-VI transition metals taken singly or in combination. The lattice structure of this matrix provides good mechanical and chemical stability, serving to inhibit migration of metal domains. As a result, cells incorporating these materials manifest very low first cycle losses. However, it seems that this approach would require a large fraction of a high-density ceramic matrix, hence would significantly reduce the specific capacity (per unit anode weight).
In summary, the prior art has not demonstrated a composite material that has all or most of the properties desired for use in an anode for the lithium-ion battery. Thus, there is a need for a new anode for lithium-ion batteries that has a high cycle life, high reversible capacity, and low irreversible capacity. There is also a need for a method of readily or easily producing such a material.
It may be further noted that the cathode materials used in the prior art Li ion batteries are not without issues. As a matter of fact, a practical specific capacity of a cathode material has been, at the most, up to 200 mAh/g, based on per unit weight of the cathode material. The positive electrode active material is typically selected from a wide variety of lithium-containing or lithium-accommodating oxides, such as manganese dioxide, manganese composite oxide, nickel oxide, cobalt oxide, nickel cobalt oxide, iron oxide, vanadium oxide, and iron phosphate. The positive electrode active material may also be selected from chalcogen compounds, such as titanium disulfate or molybdenum disulfate. These prior art materials do not offer a high lithium insertion capacity and this capacity also tends to decay rapidly upon repeated charging and discharging. In many cases, this capacity fade may be ascribed to a loss of electrical contact of the cathode active material particles with the cathode current collector.
In most of the prior art cathodes, a significant amount of a conductive material, such as acetylene black, carbon black, or ultra-fine graphite particles, must be used to improve the electrical connection between the cathode active material (typically in a fine powder form) and a current collector (e.g., Al or Cu foil). Additionally, a binder is normally required to bond the constituent particles of both the cathode active material and the conductive additive for forming an integral cathode body. The binder is typically selected from polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene-propylene-diene copolymer (EPDM), or styrene-butadiene rubber (SBR), for example. A typical mixing ratio of these ingredients is 75 to 90% by weight for the positive electrode active material, 5 to 20% by weight for the conductor agent, and approximately 5% by weight for the binder. This implies that the cathode typically contains a significant proportion of non-electro-active materials that do not contribute to the absorption and extraction of Li ions.
In addition to these two issues, conventional cathode materials also have many of the aforementioned problems associated with the anode materials. Therefore, a further need exists for a cathode active material that has a high specific capacity, a minimal irreversible capacity (low decay rate), and a long cycle life.